So far we have seen that everything in the Universe
is structured and organized. From the moons which orbit the
planets, to the Solar System itself where everything orbits the
Sun. The force responsible for this organization is gravity.

The power of gravity causes matter to attract,
pulling together into stars, worlds and planetary systems.
Once we leave the Solar System, does this complex organization
end? No, it does not. This same gravity that makes the
Solar System's worlds orbit the Sun, causes billions of stars to
clump together, orbiting in giant circles around a common center.
These collections of stars and gas are called galaxies.

Think of a galaxy as a giant Solar System.
Instead of planets circling a sun, there are stars and dust clouds
circling the center. In the Solar System, the Sun's strong
gravity holds onto the planets. It can do this because it is
much bigger than the planets are. What do you think might be in
the center of a galaxy that could be big enough to hold onto all the stars?

The answer is a very complicated one. At the
center of most galaxies we find two important things that hold onto
the galaxy's stars. Firstly, there are many, many stars packed in
tightly. If you look at a picture of a galaxy, often the center
glows very brightly. This is caused by the glow of so many
stars so close together. Secondly, most galaxies also have what is called
a supermassive black hole in their centers. These black holes
can contain as much matter as a billion stars. But all of that
matter is squished up into a space smaller than a pinhead.